Ibrahim Farah and Alex Evans both started the match while Nat Jarvis and trainee Joe Ralls came off the bench. Jarvis marked the occasion with his first senior goal.
Cardiff won the Carling Cup tie 3 - 1 after extra time.
“It’s the first time they have dipped in and played against league opposition,” said the boss. “I’m delighted for them as long as they learn from this.
“Nat scored his goal and I have told him the more he runs the more chances he will get.
“Alex was outstanding as the game wore on and Ibby has great energy.
“He lacks a little in direction and knowing about the game, but those
“Joe is another one coming through and now it’s about ensuring they all get the right education.
“A lot of Championship sides might have come here to Oxford and been turned over,” said Mackay after Craig Conway, Peter Whittingham and Jarvis had hit the target.
“I was delighted with the application and hard work.
“There are times when we could be a little tidier and better in our decision-making, but a lot of these guys had not played together before.
“They came through a test with a lot of passion.”